Recent advances in autocollimator calibration and optimisation at the PTB

Autocollimators (AC) are versatile optical instruments for the contact-less measurement of the inclination angles of reflecting optical surfaces. They are applied in optics, angle metrology, and precision engineering (form measurement of flat and aspherical optics, angular adjustment of components, calibration of angle encoders and artefacts). A comprehensive overview of the current status of AC characterisation, calibration, and optimisation at the Physikalisch-Technische Bundesanstalt (PTB) and its implications for practical applications are provided. Information on the development of our Solid Angle Autocollimator Calibrator (SAAC) will be presented. Its aim is to extend traceable angle calibrations from the plane to the solid angle - which has not yet been realised by any metrology institute - and to calibrate the effects of optical path length changes of the AC beam. Further topics include the ray-tracing modelling of ACs, the realisation and experimental testing of novel phase-shifting reticles to improve AC performance at small apertures down to 1.5 mm, algorithms for the evaluation of the reticle image on the CCD, and THz ACs.